In a conventional hydraulic circuit of this type, as shown in FIG. 2 of Japanese Utility Model Publication No. 57-72334, the oil supply line for supplying oil to the oil chamber acts also as an oil exhaust line for draining oil from the oil chamber. The oil is therefore drained from the oils chamber through a drain port of a control valve mounted on the oil supply line for switching change speed states.
Since the oil supply line acts also as the oil exhaust line, this line includes a check valve or the like. Thus, the oil drained from the oil chamber flows through the check valve and the control valve. As a result, a considerable pressure loss occurs at the check valve and the control valve. When the oil is at a low temperature and at a low viscosity, the oil tends to remain in the oil chamber without being sufficiently drained therefrom.
Sufficient drainage cannot be achieved particularly where the supply/exhaust line is long between the oil chamber and control valve or when a change speed operation is carried out within a short time. This means that a subsequent pressure buildup function is started with the oil remaining in the oil chamber. As a result, the clutch is suddenly engaged under high pressure, thereby producing a shock during a change speed operation.